Field of the Invention
This invention relates to a method for removing gaseous inclusions from viscous liquids. More particularly, this invention relates to a method for removing gaseous inclusions from high viscosity molten materials, such as molten glass.
Description of Related Art
There are a variety of commercially important materials which exist as viscous liquids during processing and contain a substantial number of gaseous inclusions. Exemplary of such viscous liquids is molten glass. New methods for melting materials such as glass have been proposed in the form of compact, high-intensity melters which employ, for example, submerged combustion melting, plasma melting, electric melting, or other means. These new melters offer significant operations and cost benefits in industrial practice, but they may produce a molten material having large quantities of gaseous inclusions. As a result, the advantages of such high intensity melters are mitigated if removal of the gaseous inclusions is slow or requires large, costly equipment.
Quality specifications for solid products produced from molten materials vary depending on the type of solid product but generally require the removal of gaseous inclusions over a certain diameter and may require removal of gaseous inclusions down to a specified number of inclusions per unit volume of the solid product. Because the inclusions cannot be reduced from a solid product, such inclusions must be removed while the precursor material is in the form of a viscous liquid.
For gaseous inclusions disposed in a viscous liquid, the natural tendency is for the gaseous inclusions to rise to the surface of the viscous liquid. However, the rate at which the gaseous inclusions rise to the surface of the viscous liquid is a function of the viscosity of the liquid and the size of the gaseous inclusions. That is, the rate at which the inclusions rise to the surface decreases with increases in liquid viscosity and decreases in gaseous inclusion diameters. Depending upon the liquid viscosity, conventional means for increasing the rate at which the gaseous inclusions rise to the surface frequently require large, costly furnaces which provide sufficient residence time to allow removal thereof by buoyancy alone. Thus, a method which provides for rapid removal of gaseous inclusions from viscous liquids, particularly from viscous liquids such as molten glass, is highly desirable.
Under normal gravity conditions, a gaseous inclusion, or bubble, will rise to the upper surface of a liquid. This is a consequence of the lower density of the insoluble gas. Increasingly precise mathematical descriptions have been developed which describe this well-known phenomenon in various liquids over a wide range of liquid viscosities, but a general description known as Stokes Law provides a reasonable understanding of the process. Stokes Law states that the velocity at which a bubble rises is proportional to the square of the bubble diameter, proportional to the acceleration of gravity, proportional to the difference in density between the liquid in the gas, and inversely proportional to the viscosity of the liquid. Most methods for speeding the removal of bubbles from liquids, particularly viscous liquids, take advantage of Stokes Law. Proposed and implemented methods for bubble removal have included putting the liquid under vacuum, or reduced pressure, so as to increase bubble diameter and increase bubble velocity; spinning a liquid to increase the effective gravitational constant, g, and, thus, increase bubble velocity; heating a viscous liquid with localized heating by various means, such as electrodes, burners, microwave, and the like, to decrease viscosity which leads to higher bubble velocity; injecting additional bubbles by using bubblers, adding a “fining agent”, or injecting a light gas such as helium, so as to effect bubble coalescence, which effectively increases bubble diameter and bubble velocity; passing the liquid over a substantially planar surface to create a thin layer, thereby reducing the distance through which the bubbles must travel to reach the upper surface of the liquid; using acoustic or ultrasonic energy to cause bubbles to vibrate, or to coalesce, or to be pushed toward coalescing zones or the surface to assist in removal of bubbles from a viscous liquid; and stirring the liquid, i.e. mechanically lifting liquid from the bottom toward the surface.
Each of the aforementioned methods to speed bubble removal has associated costs and limitations. Vacuum systems are costly to build and complex to operate. Centrifuges can be complex and are impractical when working with high temperature liquids, such as molten glass. Conventional heating of the liquid to lower the viscosity, whether using burners, electrodes, or microwaves, costs energy. The addition of new bubbles can lead to complexity, add cost for the gas, and does not assure complete capture of the smallest bubbles that are the most difficult to remove. Thin-film bubble removal by itself is impractical because a large surface area must be maintained without variations in temperature or flow rate and without excessive wear of the surface. Acoustic or ultrasonic approaches are promising, but they suffer from difficulties in scaling to a practical method that will work with the volumes of liquid commonly processed on an industrial scale. Stirring methods are used currently, but they are of limited utility and must be implemented with care to avoid the addition of new bubbles to the liquid.
The most straightforward approach to removing gaseous inclusions is to simply wait for them to rise to the surface. However, as previously indicated, this approach is particularly slow in viscous liquids because the bubbles can take a long time to rise and because small bubbles, in particular, can take an order of magnitude more time to rise. In addition, once the bubbles do reach the surface, they may stay on the surface for some time before they break.
One way to increase the rate of bubble rise and, thus, reduce refining time is to increase the temperature of the liquid containing the bubbles to decrease the liquid viscosity. For many liquids, molten glass being a prime example, a relatively small temperature increase produces a significant decrease in viscosity. Common practice is to heat the molten glass from above, thereby transferring heat down into the glass by radiation and convection. However, glass at the bottom of the molten bath also must be heated in order to fully clarify the glass, thereby necessitating overheating of all the glass above the bottom of the bath. In addition, overheating of the molten glass surface in this manner results in excessive heat loss, damage to the furnace materials, and potential volatilization of components such as boron and sodium. On the other hand, heating molten glass from above is beneficial because the bubbles reaching the surface are in a lower viscosity liquid and, as a result, are much easier to break. However, this approach to heating is inefficient because breaking bubbles on the surface of the molten glass is only useful when all of the bubbles are at the surface. If the bulk glass still contains rising bubbles, then the bubbles lower in the molten glass will need additional heat to break once they reach the surface.
As indicated above, another conventional method for decreasing refining time for molten glass is to hold the glass in a shallow bed or thin film so as to decrease the distance the bubbles are required to travel before reaching the surface to break. Typical glass melters use bed depths of 30-36 inches and forehearth (refining) channels having depths of approximately 6 inches. Using a forehearth depth for refining decreases the time required to clarify the glass by a factor of five or six. This decrease is significant but comes with the penalties of greater surface area and faster flow of glass across refractory surfaces that abrade during processing. Using depths under 4 inches for refining is impractical due to increases in surface area, heat loss, and refractory wear from the high velocities of the molten glass flowing across the surface.
The benefits of employing both the conventional method of heating molten glass and the method of holding molten glass in a relatively shallow channel are cumulative. Refining times can be decreased substantially, leading to refining times for bubbles as small as 0.1 mm in diameter in the range of about 1 to 3 hours. However, such a process would be expected to be inefficient and may also suffer from evaporative loss of some volatile components. Even with the known penalties of this approach, the practice would still suffer from a much longer refining time than desired. Production rates of glass in industrial furnaces range from about 1 to 25 tons per hour; thus, a refining chamber providing a residence time of 1 to 3 hours would be prohibitively large and costly. For a refining unit to be small enough in size to be cost-effective and practical, residence times must be reduced by another factor of 3-6 so that refining times of about 20 minutes can be achieved.
Stokes Law describes the buoyant behavior of bubbles in a viscous liquid. However, the effect of bubble surface tension is not described by Stokes Law. Depending upon the composition of the viscous liquid and the gas composition of the bubbles, bubble surface tension impacts bubble coalescence when the bubbles come in contact. Bubbles that coalesce tend to act like larger bubbles and, thus, rise more quickly. In molten glasses, the bubbles do tend to coalesce, such that when the molten glass is very bubbly and the large bubbles rise as a single mass, small bubbles cannot move around the larger bubbles and are forced to attach to the larger bubbles and rise with them. A process that provides a means for all the bubbles to rise in a single mass is, thus, much more effective at removing even the smallest of bubbles from liquid.